Marginal ROAS

Measured as

Marginal ROAS = incremental revenue / incremental ad spend

Misused when

• Scaling based on average ROAS rather than marginal profit.
• Ignoring margin/variable costs (revenue-only ROAS can mislead).

Operator takeaway

• Use marginal ROAS (or incremental profit) to decide when to scale or cut spend.
• Average ROAS can remain high even when marginal ROAS falls below break-even.
• Marginal ROAS is best estimated from experiments or response curves, not attribution alone.
• Use Marginal ROAS only inside a stable attribution rule, conversion definition, and time window so campaign comparisons stay honest.
• If performance changes, check whether the metric moved for a real business reason or because the measurement setup changed underneath you.

Next decision

• Quantify the impact with Marginal ROAS Calculator if you need to turn the definition into an operating assumption.
• Read Marginal ROAS: how to scale ads with diminishing returns if the decision depends on interpretation, policy, or trade-offs beyond the raw formula.